1. Technical Field of the Invention
The present invention relates generally to an inverter apparatus, and more particularly to an improved structure of an inverter apparatus designed to change direct current to alternating current for supplying power to an electric motor which may be employed in an electric automobile.
2. Background Art
Japanese Utility Model First Publication No. 58-153592 teaches an inverter device which includes single-phase inverter circuits each composed of a high-side switch and a low-side switch connected in series with each other. A given number of the single-phase inverter circuits are connected to a pair of terminals of a DC power source through positive and negative bus lines, respectively. The inverter device further includes capacitors each connected to both power terminals of each of the switching units for restricting voltage from being induced by the inductance of circuit lines.
Japanese Utility Model First Publication No. 59-135095 discloses an inverter device composed of switching units and capacitors. Each of the capacitors is arranged so as to correspond to one of the switching units to complete a unit hasing similar circuit structure to that of the above Publication. Additionally, each of the capacitors is oriented vertically adjacent a cooling block.
Japanese Patent First Publication No. 2-294278 discloses a DC power source composed of a rectifier and a plurality of smoothing capacitors for power supply to an inverter device. The smoothing capacitors are connected in parallel with each other through circuit lines having the same inductance.
FIGS. 19 and 20 exemplify a conventional inverter device having a structure similar to the to above mentioned prior art devices. FIG. 19 is a front view which illustrates a capacitor arranged vertically in the vicinity of a cooling block. FIG. 20 is a plan view thereof.
The inverter device in FIGS. 19 and 20 includes a cooling block 1a, three switching units 3, a capacitor 4, a positive bus line 5, a negative bus line 6, a connecting conductor 50 extending from the positive bus line 5, and a connecting conductor 60 extending from the negative bus line 6.
With the arrangements shown, the prior art inverter device cannot provide for a sufficient reduction in voltage (surge voltage) induced by the inductance of the connecting conductors 50 and 60 in a high-speed switching operation. A great reduction in the induced voltage is necessary to protect the elements of the inverter device. Of course, it is possible to provide a snubber circuit to decrease the induced voltage, but this complicates the circuit structure. This is so because the capacitor 4 and the switching units 3, as can be seen in FIG. 19, have different terminal heights, thus requiring the long L-shaped connecting conductors 50 and 60.